1. Field of the Invention
This invention relates to a method of making a semiconductor device, and more particularly to rapid thermal annealing of a semiconductor substrate by radiation of incoherent light.
2. Description of the Related Art
In a manufacturing process of a semiconductor device, rapid thermal annealing (hereinafter, referred to as RTA) of a semiconductor substrate has been used. The advantages of RTA lie in: first, since rapid heating and cooling are performed, it is possible to heat-treat a semiconductor substrate for a short time with an accuracy of second unit; and second, since semiconductor substrate is transferred to and from a heating furnace at a low temperature near the room temperature, an atmosphere of the heating furnace is accurately controlled during heating. In the heat-treatment of semiconductor substrates using an ordinary electric furnace, outside air enters into the furnace due to entrainment with transfer of the semiconductor substrate into the furnace. Accordingly, a problem of oxidizing the substrate may often occur, which should not occur. RTA is especially effective to avoid this problem, because RTA enables the heat-treatment in the inert gas atmosphere under the condition of the residual oxygen concentration being as low as almost zero. Conventionally, in the heat-treatment by RTA, a low concentration of residual oxygen has been considered as an advantage in controlling the atmosphere.
The application of RTA includes the activation of ion implantation layers, the elimination of crystal defects in the diffused layers, and the formation of resistor layers with low resistance by heat-treating a semiconductor substrate, having a region containing a high concentration impurity, in an atmosphere of inert gas such as nitrogen. In such applications, it is possible to heat-treat the semiconductor substrate without oxidation.
In the actual manufacturing process semiconductor devices, however, a problem arises in heating an exposed substrate. Particularly, when a semiconductor region having an impurity concentration of 1.0.times.10.sup.19 cm.sup.-3 or more and formed in a semiconductor substrate is heat-treated by RTA in a nitrogen atmosphere, impurity atoms and atoms constituting the substrate will be removed from the surface of the exposed semiconductor region. This results in a nonuniform sheet resistance of the semiconductor region after the heat-treatment by RTA, which makes it difficult to form a low resistance element in the semiconductor substrate. The nonuniformity of the sheet resistance results inevitably from heat-treatment of an exposed substrate, but not from variation in the temperature of heat-treatment. Additionally, in a nitrogen atmosphere, heat-treatment at as high as 1100.degree. C. or more will create surface roughness in the substrate regardless of impurity concentration. Such nonuniformity of the sheet resistance and surface roughness (hereinafter, generically referred to as damage to the substrate surface) have an undesirable effect on the uniformity and reproducibility in the manufacturing process of semiconductor devices.